Tumors arisen in F1 hybrids of two congenic lines differing in the major histocompatibility complex of the mouse (H-2), when transplanted into parental lines, produce variants that are selectively compatible with one but incompatible with the other parent. The main objective of this study is to determine the mechanism leading to the tumor-variant formation and the role of the individual H-2 regions in this process. The research goals for the current year are: to determine transplantation characteristics of tumors obtained in selected F1 hybrids; to isolate variants from transplantable tumors; to map precisely I region loci involved in tumor rejection; to determine the role of Ia antigens in tumor rejection. BIBLIOGRAPHIC REFERENCES: Klein, J., Chiang, C.L., and Hauptfeld, V. Histocompatibility antigens controlled by the I region of the murine H-2 complex. II. K/D region compatibility is not required for I-region cell-mediated lymphocytotoxicity. J. Exp. Med. 145: 450-454, 1977. Klein, J., Geib, R., Chiang, C., and Hauptfeld, V. Histocompatibility antigens controlled by the I region of the murine H-2 complex. I. Mapping of H-2A and H-2C loci. J. Exp. Med. 143:1439-1452, 1976.